In recent years, games which are played by installing a game program on a portable device from a server via a communication network have become common. Such games include games in which multiple players can participate (so-called “social games”). There are games wherein players can not only fight against or help each other, but are also enabled to communicate with each other.
Such known games include, for example, games (so-called “city building games”) wherein a player builds a city within a virtual space (hereinafter referred to as “game space”) provided in the game program. In city building games, players can build various facilities (such as houses, streets, ports, train stations, airports, castles, training facilities, etc.) on desired positions and create a city after their liking.